1. Field of the Invention
The present invention relates to a valve timing control system, which adjusts opening and closing timing (hereinafter, referred to as the valve timing) of at least one of an intake valve and an exhaust valve of an internal combustion engine.
2. Description of Related Art
For example, as recited in Japanese Unexamined Patent Publication No. 2006-46315 (corresponding to U.S. Pat. No. 7,182,052), a previously proposed valve timing control system includes a housing and a vane rotor. The housing receives a drive force of a crankshaft of the internal combustion engine, and the vane rotor is received in the housing and transmits the drive force of the crankshaft to the camshaft. The valve timing control system employs the pressure of a working fluid in a retard chamber and an advance chamber to drive the vane rotor to rotate toward the retard side or the advance side relative to the housing. In this way, the phase of the camshaft relative to the crankshaft is adjusted, i.e., the valve timing is adjusted.
When the intake valve or the exhaust valve is opened or closed in such a valve timing control system, the torque fluctuation, which is received by the camshaft from the intake valve or exhaust valve, is conducted to the vane rotor. Thus, the vane rotor receives the torque fluctuation toward the retard side and the advance side relative to the housing.
For example, in the case where the working fluid is supplied to the advance chamber to change the phase of the camshaft relative to the crankshaft from the retard side to a target phase on the advance side, when the vane rotor receives the torque fluctuation toward the retard side, the vane rotor receives the torque fluctuation in the direction for causing a reduction in the volume of the advance chamber. Thus, the working fluid in the advance chamber receives the force that causes discharge of the hydraulic fluid from the advance chamber. When the vane rotor receives the torque fluctuation toward the retard side during the advance control operation, the pressure of the working fluid in the advance chamber cannot overcome the torque fluctuation. This causes the vane rotor to be pushed back toward the retard side due to the torque fluctuation as shown by the dotted line of FIG. 13. This may result in a longer response time before achievement of the target phase. This disadvantageous phenomenon becomes particularly prominent when the pressure of the working fluid, which is supplied from a fluid source, is relatively low.
In view of this, as recited in Japanese Unexamined Patent Publication No. 2006-46315, it is conceivable to provide a check valve in a supply passage, which supplies the working fluid to the advance chamber, to limit the discharge of the working fluid from the advance chamber even when the vane rotor receives the torque fluctuation in the retard control operation. In this way, as indicated by a solid line in FIG. 13, it is possible to limit the returning of the vane rotor to the side opposite from the target phase relative to the housing in the phase control operation, and thereby the response in the phase control operation can be improved.
In the case where the check valve is provided in the supply passage, which supplies the working fluid to the retard chamber, when the advance chamber is filled with the working fluid, the discharge of the working fluid from the retard chamber can be limited even when the vane rotor receives the torque fluctuation toward the retard side in the relatively low pressure state of the working fluid.
However, in the case where the pressure of the working fluid supplied from the fluid source is relatively low, and the quantity of the working fluid supplied from the fluid source is relatively small, the time required to fill the advance chamber with the working fluid is lengthened at the time of, for example, supplying the working fluid to the retard chamber to execute the phase control operation toward the advance side. When the vane rotor receives the torque fluctuation toward the retard side before completion of the filling of the advance chamber with the working fluid, the vane rotor is returned toward the retard side, so that the response up to reaching of the target phase is disadvantageously reduced.